The physiologic role of vitamin D will be investigated by measuring the influence of the vitamin and its metabolites on cellular phosphate metabolism and alkaline phosphatase activity. The studies will be performed in vitro and in tissue culture to eliminate the systemic changes induced by the vitamin and the further metabolism of the specific metabolites of vitamin D. The same preparation will be used to study the role of the serum and cytoplasmic vitamin D binding protein in mediating the action of the sterols. The influence of vitamin D on phosphate transport will be investigated primarily in intestinal explants and isolated intestinal mucosal cells maintained in culture. The influence of vitamin D on muscle phosphate transport and phosphate homeostasis will be studied in vitro and in vivo. These studies are designed to demonstrate the role of vitamin D in the maintenance of cell function and growth.